1. Field of the Invention
The present invention relates to a filter circuit and more specifically to a filter circuit having a glass delay line, which is used in a video signal processing circuit for television receivers, video tape recorders, and the likes.
2. Description of Related Art
In a conventional filter circuit incorporating therein a glass delay line, an video signal is supplied from an input terminal through an impedance conversion buffer to one end of an input filtering capacitor, whose other end is connected through an input resistor to one end of a glass delay line. The one end of the glass delay line is also connected to a variable input coil. The other end of the glass delay line is connected to a resistor and an output coil for an output impedance matching, and an output terminal. With this arrangement, a signal having a predetermined delayed time can be obtained from the output terminal.
Now, operation will be described assuming that the delay time of the glass delay line corresponds to one horizontal scanning period of an NTSC (National Television System Committee) signal, namely to about 63.56 microseconds.
The video signal (NTSC signal) supplied from the input terminal is inputted to the impedance conversion buffer, where it is converted to a low impedance. An output of the buffer is supplied to the filtering capacitor, where a signal component from a direct current to several hundreds KHz is attenuated. An output of the filtering capacitor is supplied to the glass delay line through an input impedance matching circuit composed of the input resistor and the variable input coil.
Here, considering an equivalent circuit of the glass delay line, it can be expressed that each of the input terminal and the output terminal is grounded through a parallel circuit of a resistor Rx and a capacitor Cx. In the case of the glass delay line giving the delay time corresponding to one horizontal scanning period (1H) of the NTSC signal, there has been used a glass delay line of Rx.apprxeq.560.OMEGA. and Cx.apprxeq.110 pF. In this case, a resistance Ri of the input resistor and a resistance Ro of the output resistor are set such that Ri=Ro=Rx=560.OMEGA., and an inductance Li of the variable input coil and an inductance Lo of the output coil are set such that Li=Lo=18 .mu.H.
Under this construction, the amount of delay from the input terminal to the output terminal is set to one horizontal scanning period (1H) of the NTSC signal by adjusting the inductance Li of the variable input coil. An output signal obtained from the output terminal is a signal composed of a color signal of the NTSC signal and its neighboring signal, by action of a limited pass band of the glass delay line. Namely, a signal of 3.58 MHz and its neighborhood can be obtained. Accordingly, a color signal delayed from the video signal inputted to the input terminal by the one horizontal scanning period (1H) of the NTSC signal is obtained from the output terminal.
In the above mentioned conventional filter circuit using the glass delay line, since the input and output impedance matching circuits include the variable input coil and the output coil, respectively, external noise signals or disturbance signals are captured by the coils. In addition, the adjustment of the coil requires a mechanical adjustment of the variable input coil, and therefore, it has been difficult to electrically adjust for example by means of a direct current voltage. Furthermore, it is also difficult to implement the filter circuit on an integrate circuit.